1. Field of the Invention
The present invention relates to electric power amplifier stages, and in particular to electronic power amplifier stages with bipolar transistors.
2. Description of the Related Art
Electronic power amplifier stages are often constructed with bipolar transistors. These are sensitive to overload, with the robustness decreasing at high operating voltages or temperatures. Furthermore, the connection of the base of the power amplifier by polar transistors plays an important role.
If no protective measures are taken, an amplifier often has to be overdimensioned unnecessarily, which mostly entails significant disadvantages regarding the costs, the amplification and the efficiency etc. The working-point adjustment, in particular, plays an important role for the operating properties of the amplifier. Moreover, the working-point adjustment is supposed to be flexible, for example adjustable via an external voltage or an external current.
FIG. 3 shows a known solution with current mirrors, which have the advantage that they work in a temperature-stable manner. A signal to be amplified, which is illustrated by a signal source 30, is coupled to a base 32a of a power amplifier 32 via a coupling capacitor 31. In the embodiment shown in FIG. 3, the power amplifier 32 is embodied as npn bipolar transistor. The amplified signal is present at a collector 32b of the power amplifier 32. An emitter 32c of the bipolar transistor is connected to a reference potential, such as the ground potential. The power amplifier transistor 32 is supplied with current from a supply voltage VCC, wherein the supply current is fed via an inductance 33 or generally via a low pass to the transistor 32. For the current control, the current mirror arrangement of FIG. 3 is provided, having, on the base side, a current source 32, an npn transistor 35, and a short line 36 connecting the base of the transistor 35 to the collector of the transistor 35. The emitter of the transistor 35 is connected to the ground potential, as it is illustrated in FIG. 3.
It is disadvantageous in the current mirror solution shown in FIG. 3 that a class C operation is difficult to realize. The class C operation, however, is particularly attractive especially for RF amplifiers, that are battery-operated, such as in a mobile telephone, owing to the high efficiency. For optimum C operation, which distinguishes itself by a small current flow angle but still sufficiently high amplification, the base operating voltage or base bias voltage would have to be relatively small and be in the range of typically 0.3 to 0.5 V, taking a temperature dependence into account. The current mirror solution shown in FIG. 3, however, only permits base operating voltages in the range of 0.6 to 0.8 V.
Furthermore, there is another difficulty in that, with npn bipolar transistors as power transistors, in the breakdown case, a current, which is to be derived in a low-ohmic manner to the reference potential, i.e. to the ground, flows from the base. This breakdown case is often contingent on high operating voltages VCC or on mismatches.